WO 2012/091786 A1 disclosed an optical delay device for elongating a beam path of a light beam. The delay device has exactly one input for coupling a light beam into an interspace and exactly one output for coupling out a light beam from the interspace. The first and second pluralities of reflective surfaces are arranged and aligned relative to one another in such a way that a superimposition of two or more optical beams at each of the optical surfaces is avoided when passing through the interspace between the input and the output.
Such an optical delay device can be used for example in a driver laser arrangement for an EUV light source, as described in US 2009/0095925 A1. The driver laser arrangement described therein has a beam source for generating pulsed laser radiation and one or more optical amplifiers for amplifying the pulsed laser radiation. The beam source of the driver laser arrangement serves for generating so-called seed pulses, which are amplified to high laser powers of several kW, if appropriate of 10 kW or more, in the optical amplifier or optical amplifiers. The laser radiation amplified by the driver laser arrangement is fed via a beam guiding device to a focusing device, which focuses the laser radiation or the laser beam in a target region. A target material is provided in the target region and undergoes transition to a plasma state upon irradiation with the laser beam and emits EUV radiation in the process.
In the case of the driver laser arrangement described above, the amplified laser radiation can be reflected for example at the target material, which may be present, e.g., in the form of tin droplets. The back-reflection generated at such a droplet passes back into the optical amplifier or optical amplifiers and passes through the gain medium present there, such that the back-reflection is also amplified in the optical amplifier or optical amplifiers. Even a weak back-reflection possibly suffices to generate, after amplification in the gain medium of the optical amplifier, a power that can damage optical or, if appropriate, mechanical components in the optical amplifier or in the beam path upstream of the optical amplifier, e.g., in the beam source.
In order to prevent the reflected, returning pulse from entering the beam source, it is possible to arrange, for example between the beam source and one of the optical amplifiers, an optical switch or a switchable diaphragm which closes the beam path of the laser beam, such that the returning laser pulse is blocked and cannot enter the beam source. Moreover, it may be advantageous to close the beam path of the laser beam between two successive pulses propagating in the direction of the target material, in order to prevent a stable optical axis from being established at which ASE (“amplified spontaneous emission”) or parasitic self-lasing (at scattering locations) may form.
Since optical switches require a certain time period to block the beam path for the laser beam after the passage of a pulse, it is advantageous to provide an optical delay device in the beam path downstream or, if appropriate, upstream of an optical switch, which optical delay device elongates the beam path of the laser beam and thus increases the travel time of the laser beam. In an optical delay device, the beam path is elongated by more than 100 m, if appropriate, which can be realized by a tight folding or by a multiplicity of reflections between the reflective surfaces. In order to ensure a sufficient elongation of the beam path, optical delay devices typically require a comparatively large structural space.